Signaling System Seven (SS7)


What is Signaling System Seven (SS7)?

SS7 is a critical component of modern telecommunications systems. SS7 is a communications protocol that provides signaling and control for various network services and capabilities. Being a layered protocol, SS7 provides various protocol levels for connection oriented and connectionless (database) signaling in fixed and mobile networks.

  • Transaction Capabilities Application Part (TCAP): TCAP is the portion of the SS7 protocol stack utilized for transport of the payload of other application processes
  • ISDN User Part (ISUP): ISUP is a form of connection oriented signaling used for call set-up

While the Internet, wireless data, and related technologies have captured the attention of millions, many forget or do not realize the importance of SS7. Every call in every Public Switched Telecommunications Network (PSTN) system is dependent on SS7. Likewise, every mobile phone user is dependent on SS7 to allow inter-network roaming. SS7 is also the "glue" that sticks together circuit switched (traditional) networks with packet-switched (IP based) networks.

Origins of SS7

Common Channel Signaling Network (CCSN) technology was introduced in the mid-1970s to improve trunk signaling (e.g. signaling for call set up involving inter-office facilities). Prior to CCSN, trunk signaling was performed via multi-frequency. After the introduction of CCSN, this form of signaling would be referred to as “in-band” signaling.

The early form of CCSN was known as Common Channel Signaling number Six (CCS6) and was used within the AT&T toll network for trunk signaling. It was also used by AT&T to provide great efficiency for their In-WATS (incoming Wide-Area Telephone Service) offering, the original version of toll-free calling, which at the time was available only to AT&T prior to portability of 800 numbers.

In the 1980s, a new CCSN protocol known as Signaling System number Seven (SS7), was developed and deployed. Telephone companies soon realized the advantages in SS7 that surpassed improvements in trunk signaling and it became the vehicle for signaling to databases and other platforms associated with enhanced services enabling the advent of intelligent networking. Variations of SS7 are now the standard through the world.

SS7 Network Elements

Networks elements involved in SS7 include the following examples:

  • Service Control Point (SCP): SCPs are usually deployed in pairs. They are the brains of the SS7 network - where service logic resides
  • Signal Transfer Point (STP): STPs are always deployed in pairs. They are the backbone of the SS7 network - routes signals to network nodes.
  • Service Switching Point (SSP): By definition, an SSP is a switch that is intelligent network capable, meaning that they have software logic and triggering necessary to invoke SS7 messages based on events as well as respond to SS7 messages received to affect call control. 

More Information

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